Hemagglutinin and neuraminidase are little protein spikes on the flu's surface that help it invade cells.

Just like handbags and shoes, the spikes come in a variety of styles. There are 16 versions of hemagluttinin and nine for neuremanidase.

Each flu picks out one style from the N rack and another from the H rack. The new bird flu in China, H7N9, covers itself in H7 and N9, while H1N1 is a bit more low-key and goes only for style No. 1 from each.

All told, there are 144 different subtypes of influenza A: from H1N1 to H9N16.

All of them exist in wild birds around the world, so they are all bird flus. But every now and then, one evolves the ability to infect other animals.

H1N1 is one of the more promiscuous. It can infect birds, people, pigs and horses. H2N2, on the other hand, only makes its way into people and birds.

But all influenza A strains are social climbers. They're always itching to try out new living quarters.

So they mutate. And they also do something even more insidious: They swap accessories.

Say a bird gets infected with two viruses: H1N1 and H2N2. When the two viruses reproduce inside the bird's cells, H1N1 can grab accessories from H2N2.

This creates a new strain that looks like H2N2 but can now infect people (because it started off as just H1N1).

That's what happened in 1957 during the Asian flu pandemic, says computational biologist Richard Goldstein, from the National Institute for Medical Research in London.

Since H2N2 had never infected people before, our immune systems had never seen these accessories and thus, we had little defense against it. The new strain ended up killing roughly 70,000 Americans.

But the virus didn't stop there. "It then picked up H3 from birds in 1968 to cause the Hong Kong flu pandemic," Goldstein tells Shots. That killed another 30,000 Americans.

What about more recent epidemics?

"The 2009 flu was a real mess," he says. "The virus had some segments from H3N2 circulating in humans from the 1968 pandemic. It had genes coming in from birds, an H1 coming from pigs and an N1 coming in from a completely different virus," he says. "They all then mixed together in pigs."

Scientists are still trying to figure out H7N9.

So far, it looks like the virus got its H from ducks, its N from migratory birds and a splash of other genetic material from chickens, immunologists reported recently in The Lancet.

But how this fashion bug found its way into people is still a mystery.